main.cc

#include <stdio.h>

#include "test.h"

int main(int argc, char ** argv)
{
	int i = globalVariable;
	
	globalVariable = 50;
	
	printf("%i\n%i\n", i, globalVariable);
	
	return 0;
}

test.cc

#include "test.h"

int globalVariable = 0;

test.h

#ifndef _EXAMPLE_H_
#define _EXAMPLE_H_

extern int globalVariable;

#endif /* end of include guard: _EXAMPLE_H_ */